What's the difference between dropping a penny and telling someone you dropped a penny? Especially if you picked it back up...
Is the only difference anything ever makes in experience, and influence on later actions? Or does it physically affect the world?
Vonnegut is pure genius. (He's an author, by the way. A funny-looking one, too.) The tagline of his novel Timequake is this:
At 2:27 p.m. on February 13th of the year 2001, the Universe suffered a crisis in self-confidence. Should it go on expanding indefinitely? What was the point?
The point is, the universe shrinks ten years. Now, the universal theory a la mode is that eventually black holes will expand so much that it will draw everything in the universe into one point and the whole shebang will start over again. I didn't explain that very well. Anywho. The universe is expanding and then it will shrink.
So. If the universe shrinking means time goes backwards, and if the universe is GOING to shrink, that must mean in a couple billion millennia or so we will live our lives again-- backwards. Like Merlin-- reading the end of the book before finding out who the main character is, building the roof before the foundation. But if we could do something different the next time around... a differently shaped roof would require a much different foundation. We'll commit suice before we're born! The entire world will collapse in a sea of chaos! Ach, we'll cross that bridge when we get to it. We'll have to.
Maybe what we do make a difference in the short run. Maybe it'll affect some things for the next couple hundred years or so. But after the next Big Bang, no one will even know you ever existed.